Researchers have found that two experimental Zika vaccines given during pregnancy can protect unborn foetus against infection and birth defects.

Although rapid and promising progress on developing vaccines has been made with animal models, the finding, published in the journal Cell, is the first to demonstrate that potential vaccines could protect a foetus from the Zika virus.

"In the study, we were the first to show that two different potential vaccines given to the mother prevent the Zika virus from infecting the foetus during pregnancy in a mouse model," said senior author Pei-Yong Shi, professor at University of Texas Medical Branch at Galveston, US.

"Based on these data, we believe that evaluating the vaccines' ability to prevent birth defects in humans is warranted," Shi said.

While a Zika infection typically results in mild or symptom-free infections in healthy adults and children, the risk of microcephaly and other diseases in a developing foetus is an alarming consequence that has created a worldwide health threat.

Pregnant women who are infected with the Zika virus but never display any disease symptoms may still give birth to a baby with microcephaly, a birth defect where a baby's head is smaller than expected.

The researchers including those from the National Institute of Allergy and Infectious Diseases (NIAID), part of the US National Institutes of Health (NIH) and other partners tested a live-attenuated vaccine candidate developed by University of Texas Medical Branch and an mRNA vaccine candidate developed by the biotechnology company Moderna.

Researchers gave 20 non-pregnant female mice two injections of the mRNA vaccine candidate 28 days apart and gave a separate group of 20 non-pregnant mice two injections of a placebo at the same time points.

Another group of 18 female mice received a single immunisation with the live-attenuated vaccine while a control group of 11 mice received a placebo injection.

Subsequent tests revealed that mice that received mRNA experimental vaccine or live-attenuated experimental vaccine produced high levels of neutralising antibodies against Zika virus in their blood.

All female mice were then mated and infected with Zika virus. One week after infection, the scientists observed that most foetuses in the vaccinated mice showed no evidence of having Zika virus transmitted to them from their pregnant mothers.

Those mice that were infected showed markedly diminished levels of Zika virus RNA in maternal, placental and fetal tissues compared to placebo-injected mice, resulting in protection against placental damage and fetal demise.

Together the data showed that both experimental vaccines can restrict in utero transmission of Zika virus in mice.

Further evaluation of the experimental vaccines is warranted, the authors said, as a vaccine that prevents congenital Zika syndrome in people is a critical public health need.

"Having a Zika vaccine that can protect pregnant women and their unborn babies would improve public health efforts to avoid birth defects and other effects of the disease in regions where Zika is circulating," Shi added.

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